Optical semiconductor parts such as light-emitting diodes (LED) have many advantages including small size, efficiency, vivid color emission, elimination of bulb failure, excellent drive characteristics, resistance to vibration, and resistance to repeated turn-on and off. These parts are thus often utilized as various indicators and light sources. Optoelectronic devices using optical semiconductor parts are enclosed in cases or packages, which are now typically made of polyphthalamide (PPA) resins.
The current rapid advance of the photo-semiconductor technology has brought about photo-semiconductor devices of increased output and shorter wavelength. Photo-semiconductor devices are often encapsulated or encased using prior art PPA resins as colorless or white material. However, these encapsulants and cases are substantially degraded during long-term service and susceptible to visible color shading, separation and a lowering of mechanical strength. It is desired to overcome these problems effectively.
More particularly, JP 2,656,336 discloses that an optoelectronic device is encapsulated with a B-staged epoxy resin composition, in the cured state, comprising an epoxy resin, a curing agent, and a cure promoter, the components being uniformly mixed on a molecular level. As to the epoxy resin, it is described that bisphenol A epoxy resins or bisphenol F epoxy resins are mainly used although triglycidyl isocyanate and the like may also be used. In examples, a minor amount of triglycidyl isocyanate is added to the bisphenol A or F epoxy resin. The present inventors have empirically found that this B-staged epoxy resin composition for semiconductor encapsulation tends to yellow when held at high temperatures for a long period of time.
Triazine derivative epoxy resins are used in LED-encapsulating epoxy resin compositions as disclosed in JP-A 2000-196151, JP-A 2003-224305, and JP-A 2005-306952. None of these patents succeed in solving the problem of yellowing during long-term service at high temperature.
JP-A 2006-140207 discloses a light reflective resin composition having a high thermal conductivity and a light reflectance of at least 80% in the wavelength region of 800 to 350 nm. Since this composition is based on an epoxy resin, it suffers from the problem that yellowing can occur during long-term service at high temperature, particularly when the LED to be encapsulated in the composition is a UV, white, blue or similar LED of the high intensity type.
JP-A 2006-077234 describes a LED-encapsulating resin composition comprising an organopolysiloxane having a weight average molecular weight of at least 5×103 and a condensation catalyst. Since this organopolysiloxane must be transparent and liquid at room temperature, the composition does not lend itself to transfer molding and compression molding.
The heat released from LED is one factor that can degrade transparent encapsulants and reflectors used in LED packages and thus causes a loss of intensity or luminance. There is a need for a material having a high reflectance and a high thermal conductivity or heat resistance.
At the present, cases for optoelectronic parts such as LED are generally manufactured by mounting a LED chip on a lead frame, wire bonding, molding an epoxy or silicone resin thereon for encapsulation, and further molding a reflector material thereon to form an enclosure or reflector for preventing any escape of light from LED. The resin composition for enclosure must be adherent to the lead frame. Conventional enclosing resins, however, are less adherent to lead frames. Specifically, in the mounting process including a solder reflow step, the assembly is exposed to such high temperatures (215 to 260° C.) that separation may occur at the interface between the lead frame and the resin. Lack of reliability is a serious problem.